Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Net interest income (loss)	$ 5,435	$ 5,588	$ 15,490	$ 16,332
Benefit for credit losses	673	1,550	3,458	1,050
Net interest income (loss) after benefit for credit losses	6,108	7,138	18,948	17,382
Guaranty fee income	23	31	78	99
Investment gains (losses), net	467	299	934	1,155
Fair value gains (losses), net	(491)	(2,589)	(4,971)	(1,902)
Gains (losses) from partnership investments	(12)	(5)	(9)	235
Fee and other income (expense)	152	228	474	1,024
Administrative expenses	(661)	(952)	(2,027)	(2,364)
Foreclosed property income (expense)	(110)	(497)	(507)	(1,152)
TCCA fees	(465)	(413)	(1,358)	(1,192)
Other income (expenses), net	(288)	(210)	(809)	(647)
Income (loss) before federal income taxes	4,723	3,030	10,753	12,638
Provision for federal income taxes	(1,527)	(1,070)	(3,475)	(4,150)
Net income (loss) attributable to Fannie Mae	3,196	1,960	7,278	8,488
US Treasury [Member]
Segment Reporting Information [Line Items]
TCCA fees	(465)	(413)	(1,358)	(1,192)
Business Segments [Member] | Single-Family [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)	166	66	485	93
Benefit for credit losses	645	1,545	3,404	967
Net interest income (loss) after benefit for credit losses	811	1,611	3,889	1,060
Guaranty fee income	3,305	3,145	9,787	9,277
Investment gains (losses), net	0	(1)	(1)	(2)
Fair value gains (losses), net	0	(4)	0	(8)
Gains (losses) from partnership investments	(17)	(12)	(54)	(27)
Fee and other income (expense)	89	98	290	571
Administrative expenses	(479)	(649)	(1,483)	(1,591)
Foreclosed property income (expense)	(114)	(516)	(510)	(1,183)
Other income (expenses), net	(382)	(180)	(1,046)	(669)
Income (loss) before federal income taxes	2,748	3,079	9,514	6,236
Provision for federal income taxes	(808)	(1,040)	(2,544)	(2,040)
Net income (loss) attributable to Fannie Mae	1,940	2,039	6,970	4,196
Business Segments [Member] | Single-Family [Member] | US Treasury [Member]
Segment Reporting Information [Line Items]
TCCA fees	(465)	(413)	(1,358)	(1,192)
Business Segments [Member] | Multifamily [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)	(22)	(16)	(62)	(73)
Benefit for credit losses	28	5	54	83
Net interest income (loss) after benefit for credit losses	6	(11)	(8)	10
Guaranty fee income	431	367	1,216	1,064
Investment gains (losses), net	23	5	37	29
Fair value gains (losses), net	0	0	0	0
Gains (losses) from partnership investments	5	7	45	262
Fee and other income (expense)	49	58	156	193
Administrative expenses	(84)	(109)	(254)	(280)
Foreclosed property income (expense)	4	19	3	31
Other income (expenses), net	(10)	5	(21)	(8)
Income (loss) before federal income taxes	424	341	1,174	1,301
Provision for federal income taxes	(49)	(17)	(127)	(128)
Net income (loss) attributable to Fannie Mae	375	324	1,047	1,173
Business Segments [Member] | Capital Markets [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)	1,049	1,401	3,221	4,516
Benefit for credit losses	0	0	0	0
Net interest income (loss) after benefit for credit losses	1,049	1,401	3,221	4,516
Guaranty fee expense	(189)	(210)	(576)	(658)
Investment gains (losses), net	2,232	1,608	5,735	4,679
Fair value gains (losses), net	(530)	(2,697)	(5,063)	(2,112)
Gains (losses) from partnership investments	0	0	0	0
Fee and other income (expense)	69	83	121	288
Administrative expenses	(98)	(194)	(290)	(493)
Foreclosed property income (expense)	0	0	0	0
Other income (expenses), net	(44)	(1)	(103)	(12)
Income (loss) before federal income taxes	2,489	(10)	3,045	6,208
Provision for federal income taxes	(670)	(13)	(804)	(1,982)
Net income (loss) attributable to Fannie Mae	1,819	(23)	2,241	4,226
Reconciling Items [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)	4,242	4,137	11,846	11,796
Benefit for credit losses	0	0	0	0
Net interest income (loss) after benefit for credit losses	4,242	4,137	11,846	11,796
Guaranty fee expense	(3,524)	(3,271)	(10,349)	(9,584)
Investment gains (losses), net	(1,788)	(1,313)	(4,837)	(3,551)
Fair value gains (losses), net	39	112	92	218
Gains (losses) from partnership investments	0	0	0	0
Fee and other income (expense)	(55)	(11)	(93)	(28)
Administrative expenses	0	0	0	0
Foreclosed property income (expense)	0	0	0	0
Other income (expenses), net	148	(34)	361	42
Income (loss) before federal income taxes	(938)	(380)	(2,980)	(1,107)
Provision for federal income taxes	0	0	0	0
Net income (loss) attributable to Fannie Mae	$ (938)	$ (380)	$ (2,980)	$ (1,107)